1. Field of the Invention
The present invention relates to a servo control apparatus and method for a video cassette tape recorder (VCR), and in particular to an improved servo control apparatus and method for a VCR capable of minimizing jitters caused in a reproducing mode of the VCR by detecting a duty interval of a reproduced horizontal synchronous signal and controlling the rotation speed of a drum motor using the thusly detected duty interval.
2. Description of the Conventional Art
Generally, as well known to those skilled in the art, the servo control system of a VCR is classified into a drum servo for controlling a drum motor and a capstan servo for controlling a capstan motor. The capstan servo system is directed to transferring tape in a horizontal direction with a vector Vc, and the drum servo system is directed to controlling the rotation of the drum which rotates at a certain angle about a tape surface. FIG. 1 shows a conventional video tape track representing a tape run by the capstan servo system and the drum rotation which is controlled by the drum servo system.
FIG. 2 shows a conventional servo system of a VCR. The control loop drives the drum motor 1 by a driving force of a drum motor driving unit 15. A magnetic sensor 4 detects the number of rotations of the drum motor 1 with a rotary sensor 2. As shown in FIG. 3A, a certain detection signal which corresponds to the rotation of the drum motor 1 is output from the rotary sensor 2 and converted into a detection signal FG, having a wave form as shown in FIG. 3B, by a second pulse shaping unit 10. The detection signal FG is output to a speed driving signal generator 11.
As shown in FIG. 3C, the speed driving signal generator 11 resets a counter (not shown) at a leading edge of a detection signal FG, and outputs the counter value at the leading edge of the detection signal FG to obtain the wave form "c" shown in FIG. 3D. The speed driving signal of the speed driving signal generator 11 is inputted to a speed/phase control value adder 13 through a first filter 12. The output signal thereof is converted into an analog signal by a D/A converter 14, and is outputted to a drum motor driving unit 15. The drum motor driving unit 15 outputs a driving output corresponding to its input signal thereby driving the drum motor 1.
Meanwhile, regarding the phase control loop of the drum motor 1, a magnetic sensor 3, which is directed to detecting the phase of the drum motor 1, outputs a pulse with respect to the rotation of the drum motor 1 as shown in FIG. 3E. The output from the magnetic sensor 3 is converted into a rectangular wave form, a phase detection signal PG, by a first pulse shaping unit 5 as shown in FIG. 3F, and outputted to the phase driving signal generator 6. The phase driving signal generator 6 receives the output of the first pulse shaping unit 5 and a reference signal ref, as shown in FIG. 3G, which is output from a switch 9. The phase driving signal generator 6 performs a certain count operation as shown in FIG. 3H. That is, the phase driving signal generator 6 resets a counter (not shown) whenever the reference signal ref is inputted thereto, and outputs the count value as a phase driving signal "f", as shown in FIG. 3I, whenever the phase detection signal (See FIG. 3F) is input thereto. The reference signal ref of the phase driving signal generator 6 varies in accordance with a recording mode or a reproducing mode. In a recording mode, a vertical synchronous signal Vsync is inputted as a reference signal ref as shown in FIG. 3J, and in a reproducing mode, a signal "i", shown in FIG. 3L, outputted from the reference signal generator 8 is inputted as a reference signal.
The switch 9 is switched in accordance with a recording/reproducing mode signal REC/PB outputted from a system controller (not shown), and determines a reference signal which is applied to the phase driving signal generator 6. The reference signal generator 8 includes a free run counter having a cycle Vsync of a TV as shown in FIG. 3K, and outputs a reproducing reference pulse "i" when an overflow occurs by the counter.
The phase driving signal "f" outputted from the phase driving signal generator 6' is outputted to the speed/phase control value adder 13 through a second filter 7, added to the speed control value outputted from the first filter 12, and converted into an analog value by the D/A converter 14, The thusly converted analog voltage is amplified by the drum motor driving unit 15 for driving the drum motor 1.
Although the drum and capstan motors are accurately controlled using the above-described control loops, when a tape recorded by a certain VCR set is reproduced by another VCR set, jitters occur because of the following reasons: First, the surface of the drum differs on each VCR set. Second, there is a load difference for the load of the tape running mechanism. Third, the speed difference of the drum motor and the speed difference of the capstan motor differ from each other due to speed and phase differences between each VCR set.
The above-mentioned differences result in difference between the horizontal synchronous signals of the video signal on the tape at the time of recording by the video head H1 of the drum motor.
Therefore, apparatuses such as a time base corrector for resolving the above-mentioned problems are introduced in the industry. Such an apparatus is directed to minimizing jitters of the picture by using memory. However, since such an apparatus requires additional memory, fabrication cost is high.